(1) Field of the Invention
The invention belongs to the overall technical class of aeronautics, and air transport.
More precisely, the invention concerns the domain of aircraft structures. Among such aircraft structures, the invention relates to the technical field of constructional features of aircraft structures, such as frames, stringers, longerons, bulkheads, but also outer shells, fairings, covers or fuselage sections.
Specifically, the invention proposes a subfloor structure for a rotary wing aircraft or rotorcraft, such as a helicopter.
(2) Description of Related Art
Before discussing the background of the invention, some terms are briefly defined.
As per FIG. 1, a rotary wing aircraft 1 comprises an airframe 2, i.e. an aircraft structure that defines an outer loft 5, i.e. the aerodynamic envelope of the fuselage. The airframe 2 is extended longitudinally and laterally aside an anteroposterior plane XZ of the aircraft 1. Such an aircraft 1 is having structural constructions, including a subfloor structure 3 at a lower part LP of the airframe 2. A rotary wing aircraft 1 also have one—or a plurality of—: rotors R, landing gear LG, and equipment.
Now referring to FIG. 2, is discussed a typical background of the invention.
In a classical rotary wing aircraft 1, a subfloor structure 3 is arranged within the lower part LP, between a floor surface 4 defining a cabin level and the outer loft 5. Typically, the subfloor structure 3 comprises a bottom shell 16, outer floor panels 13, inner floor panels 14 and a framework construction 15. The framework construction 15 connects the floor panels 13-14 to the bottom shell 16. For typical aircrafts, the bottom shell 16 covers most of the surface of the lower part LP of the outer loft 5, below the floor surface 4. The framework construction 15 comprises interconnected longerons 8, outer ribs 9 and inner ribs 10, as well as the lower portion of main frames 11.
The longerons 8 are structural and generally longitudinal beams being basically arranged side by side along the anteroposterior plane XZ. The longerons 8 are spanning the entire length of the subfloor structure 3, whereas the outer ribs 9 and inner ribs 10 are forming crossbeams.
Such crossbeams are basically arranged orthogonally to the anteroposterior plane XZ and span the whole width of the subfloor structure 3 at their respective location. Hence, the framework construction 15 adopts a somehow perpendicular grid configuration with numerous intersections, such as the discrete kink locations 12.
Upwardly, both the longerons 8 and the ribs 9-10 extend from the bottom shell 16 to the floor surface 4 and feature basically flat stiffened webs. Each longeron 8 or rib 9-10 has respective caps, along the upper and lower perimeter of the corresponding flat stiffened web.
Typical in such architectures, a fore portion of the longerons 8, proximal to the nose of the subfloor structure 3, is tilted inwardly with respect to a longitudinal direction X of the aircraft 1, in order to adapt the construction framework 15 to the outer loft 5 that shows a smaller width close to the front (nose) of the aircraft 1. The longerons 8 are locally attached to respective ribs 9-10 at the discrete kink locations 12, where the aft portion of the longeron 8 is substantially in line with the longitudinal direction X.
The fore portion of the longerons 8 is also upwardly slanted by a predetermined angle in the anteroposterior plane XZ, with respect to the longitudinal direction X. The caps being connected to the rib at the discrete kink locations 12, this provides for adequate support of the caps of the longerons 8, reacting the discrete deflection of the normal loads of the caps of the longerons 8. Geometrically speaking, the longerons 8 show a longitudinal trace progression described by two straight lines with no tangential continuity at their connection point.
At side portions of the bottom shell 16, it is typical having access openings 6, providing access to the systems/equipment integrated in the space between the longerons 8 and these side portions. The side portions and an essentially flat lower portion of the bottom shell 16 are forming a single part in some known airframes 2. In other known airframes 2, the side portions of the bottom shell 16 are individual parts attached to the lower portion. Known airframes 2 have a bottom shell 16 made from composites. Such bottom shell 16 typically features a sandwich design with monolithic regions all along the joints connecting the construction framework 16 to the bottom shell 16.
The tasks of the subfloor structure 3 are manifold. A typical subfloor structure 3 takes on the one hand the payload efforts and the loads from the landing gear LG. Such a subfloor structure 3 also transmits these efforts and loads to the main frames 11 globally acting, for specific types of architectures, as a beam supported by the two main frames 11.
On the other hand, most known subfloor structures 3 house various systems/equipment (e.g. electrical, mechanical and armoring). Typically, in known subfloor structures 3 housing of systems/equipment is mostly located in lateral volumes enclosed laterally between the longerons 8 and the side portions of the bottom shell 16. Close to the center of the bottom shell 16, on sides of the anteroposterior plane XZ, the longerons 8 confine, together with corresponding crossbeams formed by the ribs 9-10, compartments for fuel tanks T. In such compartments, elastomeric bladders are installed.
Known subfloor structures 3 further provide for substantial kinetic energy absorption in case of a crash scenario of the rotary wing aircraft 1. The longerons 8 are beams working as main load carrying members.
So as to illustrate the prior art, citation is now made of published documents related to constructional features of aircraft structures. For disclosure statement, the documents are US2007/0114331, US2005/0001093 and U.S. Pat. No. 6,427,945.
The document US2007/0114331 describes a helicopter collapsible deck having at least one longitudinal member and at least one cross member, which extend respectively in a first and second direction intersecting at a point; the cross member is interrupted at the point of intersection. The deck also has an anchoring device for connecting the longitudinal member and the cross member at the point of intersection. The anchoring device has at least one local permanent deformation section lying in a plane crosswise to the deck and for dissipating the energy transmitted to the deck in the event of impact.
The document US2005/0001093 describes an impact resistant structure of a helicopter, which includes: an energy absorber positioned under a floor of the helicopter and directly connected to a cabin frame of the helicopter. The energy absorber is arranged in accordance with a distribution of a ground reaction force on a general ground at a time of crash situation. Another aspect provides an energy absorber that includes: a plurality of independent hollow tubes of fiber reinforced composite material integrally formed by bundling only the hollow tubes. The hollow tubes are arranged so as to reduce a number of intersecting wall surfaces of the hollow tubes.
The document U.S. Pat. No. 6,427,945 describes a subfloor structure of an aircraft airframe of a helicopter that includes longerons and crossbeams that intersect each other and are interconnected to form a grid that is fixedly attached to the floor and the bottom skin of the aircraft fuselage. Structural elements such as pyramid frustums and reinforcements are arranged on the beams. The longeron and the crossbeam have a trapezoidal cross-section open on the wider base side, closed by a spine web along the narrow side, and bounded laterally by inclined leg webs that extend downwardly from the spine web at an angle outwardly relative to each other. The subfloor structure grid effectively absorbs the energy of a crash impact having both axial or vertical as well as non-axial or lateral impact force components.
The document US2012/0112004 describes a shock absorbing structure for a helicopter. The shock absorbing structure is miniaturized by providing a beam-like member having a recess and a shock absorbing member. One end of the shock absorbing member is arranged in the inside of the recess and the other end of the shock absorbing member is arranged outside of the recess. The area of the recess overlaps the place where the structure member supports the structure even at a dead-stroke in which the shock absorbing member is bottomed out.
The document “Composites soften impact” (in Structures by Rob Coppinger, page 26, London, 2005), referenced as XP001227208, describes composites soften impact in helicopter structures. In a subfloor, cones for energy absorption are tailored in terms of stiffness and strength, while skin beam joints are specially designed together with a toughened energy absorbing skin. From the single Figure, classical longerons are provided, as usual in many helicopter structures.
Although the prior art provides interesting techniques, technical problems remain unsolved and useful enhancements would be beneficial.
In a few words, enhancing the efficiency of a rotorcraft subfloor structure in terms of structural weight, design complexity, assembly work, manufacturing and overall production costs is becoming more and more required. This is due e.g. to the increasing cruising speed available with modern rotorcraft, the upgrade of customer needs in terms of payload/passengers capacity, the expending demand for lighter/cleaner/safer/longer-ranged and more silent apparatuses. While these constraints are growing, the demand for more energy saving and long-lasting rotorcrafts also increases, in an antagonistic manner.
Besides, some other relative drawbacks may derive from prior art, as exemplified hereafter.
Both, the longerons and the bottom shell are frequently load carrying. A load proof connection between the lower longeron caps and the bottom shell is required. This connection is highly loaded in the fuel tank area due to the peeling load excited by the fuel inertia transverse pressure load. This load proof connection requires a high reliability and strength, high accuracy during the assembly process, high damage tolerance and adequate reparability. This load proof connection is hence defined by additional structural joints and the associated increase in structural complexity and production efforts in terms of e.g. tolerance management, quality assurance, assembly, sealing and production time. This finally translates to higher structural weight and production costs.
The discrete deflection of the load of the longerons at the intersectional kink locations results in high local stress levels at the upper caps of the longerons in the connection area. The upper caps have then to be supported by the rib by means of additional brackets. This is hence increasing weight and structural complexity.
Due to the numerous cut-outs at the lateral sides of the fairings in the lower shells, most integral shell designs with integrated side shells becomes quite ineffective in terms of mechanical performance and manufacturing costs. In this case, an additional secondary element—the side shell portions with cut-outs—is integrated to a primary element—the bottom shell portion. Meanwhile a primary element—the longerons—is attached to another primary element—the bottom shell portion—by means of a highly loaded structural connection. Facing the outstanding possibilities of composites materials allowing for high structural integrability, this technical solution is deemed ineffective in terms of weight and reliability. The most current prior art rotorcrafts having alternatively separate side shells attached to a separate lower cover shell part includes another joint in the structure hence increasing complexity, assembly and production costs.
Due to the necessity to attach the longerons to the bottom shell, the bottom shell has to provide for monolithic areas along the attachment to the construction framework. Providing for monolithic areas within a sandwich shell would reduce the bending stiffness of the sandwich bay, and would increase the production time and complicate an automatic lay-up with automatic composite fiber placement techniques.
In view of the above drawbacks, it is an aim of the invention to provide for a subfloor structure that simplifies the structural arrangement of main load bearing components, in order to improve the overall structural airframe efficiency in terms of weight and production costs. Should a composite subfloor structure be contemplated, this would take benefit of the design and manufacturing advantages of composite materials. But similar benefits may be reached with other construction methods such as forged integrated large-sized metal items, e.g. involving light alloys like aluminum alloys.
Therefore, an object of the invention is to provide a subfloor structure avoiding, among others, most of the exposed drawbacks of the prior the art.
In this purpose, based upon the teachings of the document U.S. Pat. No. 6,427,945, claimed objects are a subfloor structure and a rotary wing aircraft.